1. Field of the Invention
The present invention relates to an ink-jet printing method, an apparatus and a system for printing an image by a plurality of inks including particular color, inks
2. Description of the Related Art
Recently, as an information processing device such as a personal computer or other apparatus has become prevalent, a printing apparatus which is a terminal device for forming an image has also been rapidly developed and become prevalent. Of various printing apparatuses, an inkjet printing apparatus for printing records on a printing medium such as paper, cloth, plastic sheet or OHP sheet by ejecting ink from orifices is in a main trend of personal users since it has extremely excellent features; for example, it employs a non-impact type low-noise printing system, it is capable of carrying out a high-density and high speed printing operation, it is easily applicable to a color printing and the cost thereof is reasonable.
The progress of the ink-jet printing technology facilitates the high image quality, high operational speed and decreased cost of printing, and largely contributes to the extension of the printing apparatus into personal users in association with the extension of personal computers or digital cameras (those performing the function by themselves as well as those integral with other devices such as portable type telephones). Thus, the further improvement in image quality has been required from the personal users as the ink-jet printing apparatus has become prevalent in such a manner. Recently, a printing system capable of easily printing a photograph in a home and an image grade corresponding to a silver film photograph has been demanded.
When comparing the image formed by the ink-jet printing apparatus with the silver film photograph, the former has been problematic in a peculiar granular feeling. Accordingly, various countermeasures have been recently proposed for reducing the granular feeling, and there have been also various printing apparatuses applying such countermeasures. For example, there is an ink-jet printing apparatus having an ink system including not only normal cyan, magenta, yellow and black inks but also light cyan or light magenta ink having a lower density. According to such an ink system, it is possible to reduce the granular feeling by using light cyan or light magenta in a lower optical density area. Also, it is possible to realize the wider color regeneration and smoother gradation by using the normal cyan and magenta in a higher density area.
Further, there is also a method in which a size of ink dot on a recording medium is designed to be smaller to reduce the granular feeling of the image. To realize this, a technology has also been developed for lessening an amount of ink droplet ejected from the respective printing element arranged in a printing head. In this case, it is possible to obtain a high-resolution image without decelerating the printing speed by lessening the amount of ink droplet as well as arranging more recording elements at a higher density.
Furthermore, for the purpose of obtaining a high image quality as good as or exceeding that of the silver film photograph, a printing apparatus has been proposed in which an image is printed by the addition of novel ink colors such as red (R), green (G) or blue (B) to enlarge a gamut in which the image is represented. These R, G and B colors are referred to as secondary colors originally representable by overlapping the primary colors, cyan, magenta and yellow, with each other. However, in practice, there is a tendency to decrease the mutual color development effect in a stage of overlapping these primary colors with each other on the printing medium. It has generally been known that the image having the better color development is obtained when the secondary colors are printed as single colors. Hereinafter, the ink used while laying a stress on the color development on the printing medium, such as red, green or blue, is defined as a particular color ink.
In this regard, many methods of a so-called binary digitization processing have been proposed or disclosed, for converting multi-gradation data representing the density of the printed image to binary data determining whether or not the ink droplet is printed on the printing medium, and it is basically possible to adopt any of them. However, under the circumstances wherein kinds of ink color increases or printer resolution is higher, there may be a case wherein the binary digitization processing of all colors could hardly be carried out by one means in one process. Accordingly, printing apparatuses for carrying out a two-stage binary digitization processing have been recently provided, in which, after the quantization process for reducing the gradation levels to several levels by a printer driver installed in a host apparatus connected to the printing apparatus, the final quantization processing is carried out within the printing apparatus. In this case, since the gradation of one pixel output from the host apparatus is represented by a plurality of density levels, this method is suitable for a use laying a stress on the gradation as in the photographic image quality.
Several methods have already been proposed and carried out for converting multi-density data of several levels to binary data. For example, in Japanese Patent Application Laid-open No. 9-046522 (1997), a method is disclosed, in which one input pixel having a gradation value of five levels is represented by the printing or non-printing of four dots in a 2×2 area. Further, according to this document, a plurality of patterns of the dot arrangement in the 2×2 areas are prepared for the same gradation value, and sequentially or randomly arranged. Since the dot arrangement pattern to the respective gradation value is not fixed, a so-called sweeping phenomenon, false contour appearing in an edge portion of the image or others occurs when the pseudo-intermediate tone processing is carried out. Also, it has been described that there is an effect for averaging the use of a plurality of printing elements arranged in a printing head.
Further, a conversion method is also proposed for improving the above technology as the printing apparatus is developed to be more precise. For example, according to Japanese Patent Application Laid-open No. 2002-029097, printing apparatuses are disclosed, in which a printing head having two rows of printing elements, each row being different in properties from the other while ejecting ink droplets of the same color, or a method is adopted for switching printing or non-printing in every columns to shorten the printing times (a column-thinning method). And, a method for periodically changing a method of cyclic arrangement of some dot-arrangement patterns to solve the respective trouble, while preparing a plurality of dot-arrangement patterns, each having the same gradation level but different in dot arrangement from the other.
The conventional method for converting the multi-level density data to binary data typically described in the above patent documents is effective for overcoming various troubles occurring when the printing is carried out by using a monochrome ink, but there is no consideration for solving peculiar problems occurring when multicolor inks are used. As described in BACKGROUND OF THE INVENTION, in general, when inks of different colors are overlapped with each other, a phenomenon occurs that the color development is disturbed in the respective ink. When the multicolor inks are used at the same time as in a recent case, a probability becomes higher in that different colors are overlapped with each other. Particularly, it is true to the particular color inks added to facilitate the color-development and this results in a serious problem to deny the existence thereof.